1. Field of the Invention
The present invention concerns an optical transmission line, which is used for example for wavelength division multiplexed optical transmission, a negative dispersion optical fiber to be used in the optical transmission line, and an optical transmission system which uses the optical transmission line.
2. Discussion of the Background
Generally, a single mode optical fiber (shall be referred to hereinafter as xe2x80x9cSMFxe2x80x9d) is used in an optical transmission line, and this SMF has a zero dispersion wavelength in the 1310 nm band. Though the use of this SMF for optical transmission in the 1550 nm band is being considered, the SMF has a positive dispersion value and a positive dispersion slope in the 1550 nm band. Thus when the SMF is used singly for wavelength division multiplexed optical transmission in the 1550 nm band, the ill effect of wavelength dispersion occurs. Therefore, in order to compensate for this wavelength dispersion, active research is being carried out on module-type wavelength dispersion compensation optical fibers for short length use (this type of module-type dispersion compensated optical fiber shall be referred to hereinafter as xe2x80x9cDCFxe2x80x9d). Such a DCF is disclosed for example in Japanese Laid-open Patent Publication No. Hei 6-11620.
As an example of a DCF, a DCF has been developed with which the refractive index of the center core is made high to make the dispersion value a large negative value and thereby achieve a high figure of merit (FOM) (FOM=approximately 200).
Also, known forms of refractive index profiles of DCF""s include single peak type profiles, such as those of matched cladding type fibers, and multiple cladding type fibers, such as W-type fibers. The abovementioned single peak type DCF has a positive dispersion slope. Thus when this type of DCF is connected to an SMF, though the dispersion at a single wavelength will be compensated, the dispersion slope will increase further. This type of DCF is therefore unsuitable for wavelength division multiplexed transmission (shall be referred to hereinafter as xe2x80x9cWDM transmissionxe2x80x9d). Meanwhile, W-type and other types of multiple cladding type DCF""s are optical fibers that can compensate the dispersion and dispersion slope of an SMF. These fibers have thus been attracting attention in that they provide an arrangement suitable for WDM transmission when connected with an SMF.
That is, a slope compensation type dispersion compensation fiber (DFCF), which can compensate the dispersion value and the dispersion slope at the same time, is being demanded for compensation of the wavelength dispersion of an SMF. A high FOM and the control of the compensation factor described below are required of such a DCF.
The dispersion compensation performance that is exhibited when a DCF is connected with an ordinary SMF can be understood readily when expressed by the compensation factor as follows:
Compensation factor (%)={(SDCF/SSMF)/(DDCF/DSMF)}xe2x80x83xe2x80x83(1)
In equation (1), SDCF is the dispersion slope of the DCF, SSMF is the dispersion slope of the SMF, DDCF is the dispersion value of the DCF, and DSMF is the dispersion value of the SMF. The above values are values within the bandwidth of SMF dispersion compensation by the DCF (conventionally, a bandwidth of 1520 to 1570 nm) or values at an arbitrary wavelength within this wavelength band. With regard to the above equation, wide bandwidth zero dispersion can be accomplished more successfully the closer the compensation factor is to 100%. An optimal design for this DCF is proposed in Japanese Laid-open Patent Publication No. Hei 8-136758.
However, such a DCF aimed at short lengths is effective only for dispersion compensation of SMF""s that have been installed presently and cannot comprise a new fiber line just by itself. Due to the nature of its profile, the above-described DCF cannot maintain the low nonlinearity that is the excellent feature of SMF""s. That is, a DCF is aimed at compensating the dispersion value or dispersion slope of an SMF with as short a length as possible. A DCF is thus generally small in MFD and large in xcex941, and such a DCF tends to be extremely likely to give rise to nonlinear phenomena.
Recently, line-type dispersion compensation optical fibers with dispersion characteristics that are inverse to those of the SMF (this type of line-type dispersion compensation optical fiber shall be referred to hereinafter as xe2x80x9cRDFxe2x80x9d) are being considered as optical fibers of low nonlinearity that can compensate the dispersion and dispersion slope efficiently. RDF""s are described for example in ECOC ""97 Vol.1 p.127 and Japanese Laid-open Patent Publication No. Hei 10-319920.
The above-described conventional DCF""s and RDF""s are designed only for compensation in the 1520 nm to 1570 nm band (shall be referred to hereinafter as the xe2x80x9cC-bandxe2x80x9d).
Recently, the use of a wavelength band of 1570 nm or more, or to be more specific, the use of the 1570m to 1620 nm band (shall be referred to hereinafter as the xe2x80x9cL-bandxe2x80x9d) for wavelength division multiplexed optical transmission is being considered. For example, optical amplifiers that can amplify light of the L-band are being developed. Expansion of the wavelength band of wavelength division multiplexed optical transmission by performing wavelength division multiplexed optical transmission using both this L-band and the abovementioned C-band is being considered.
However at present, dispersion compensation optical fibers for compensation in the L-band have not been proposed and optical transmission lines for performing wavelength multiplexed optical transmission in the wavelength band of the L-band have not been realized.
Though optical transmission lines that are comprised of SMF""s and dispersion compensation optical fibers have merits, such as {circle around (1)} to {circle around (3)} given below, since such transmission lines were targeted mainly at the C-band, they are not suitable as L-band wavelength division multiplexed optical transmission lines. The abovementioned merits include the following: {circle around (1)} SMF of low nonlinearity and low loss can be used. {circle around (2)} The dispersion in the C-band becomes flat. {circle around (3)} Since the line has a large local dispersion (dispersion value per unit length), the occurrence of four-wave mixing (shall be referred to hereinafter as xe2x80x9cFWMxe2x80x9d), which becomes prominent near zero dispersion, can be restricted.
FIG. 13 is a conceptual diagram of the dispersion characteristics of the condition where an SMF is connected to an RDF for C-band compensation. As shown in FIG. 13, an optical transmission line, in which an SMF is connected to a C-band compensation RDF, has a large negative dispersion and dispersion slope in the L-band. Thus when an L-band optical signal is transmitted through an optical transmission line for the C-band, the distortion of the signal waveform due to dispersion becomes a large obstacle that makes WDM transmission in the L-band difficult. The same can be said for a DCF for C-band compensation.
An optical transmission line and an optical transmission system, with which wavelength division multiplexed optical transmission can be performed using both wavelength bands of the L-band and the C-band as mentioned above, were thus difficult to realize.
The present invention has been made to solve the above problems. That is, a final object of this invention is to present a wavelength division multiplexed optical transmission system with which wavelength division multiplexed optical transmission can be performed using both wavelength bands of the L-band and the C-band. In order to achieve this purpose, this invention first provides an optical transmission line that enables high-quality wavelength division multiplexed optical transmission in the L-band and an optical transmission line that enables high-quality wavelength division multiplexed optical transmission in the C-band. In order to realize these optical transmission lines, this invention provides a negative dispersion optical fiber that enables dispersion compensation of an SMF or other positive dispersion optical fiber in a preset wavelength band.
An optical transmission line of the first arrangement of this invention is characterized in that a positive dispersion optical fiber, with which both the dispersion value and dispersion slope in a preset wavelength band within a wavelength band of 1570 to 1620 nm are positive, is connected to a negative dispersion optical fiber, which compensates the dispersion and dispersion slope of the abovementioned positive dispersion optical fiber in the abovementioned preset wavelength band to make the dispersion value of the optical transmission line as a whole greater than or equal to xe2x88x921 ps/nm/km and less than or equal to 1 ps/nm/km within the abovementioned preset wavelength band.
An optical transmission line of the second arrangement of this invention is characterized in that a positive dispersion optical fiber, with which both the dispersion value and dispersion slope in a preset wavelength band within a wavelength band of 1570 to 1620 nm are positive, a negative dispersion optical fiber, which compensates the dispersion and dispersion slope of the abovementioned positive dispersion optical fiber in a wavelength band adjacent to the wavelength band of 1570 to 1620 nm, and a dispersion characteristics adjustment fiber, which compensates the dispersion and dispersion slope in the abovementioned preset wavelength band of the optical fiber :connection unit formed by connection of the abovementioned negative dispersion fiber and the abovementioned positive dispersion optical fiber, are connected to make the dispersion value of the optical transmission line as a whole greater than or equal to xe2x88x921 ps/nm/km and less than or equal to 1 ps/nm/km within the abovementioned preset wavelength band.
An optical transmission line of the third arrangement of this invention is characterized in that, in addition to having the above-described second arrangement, the wavelength band adjacent the wavelength band of 1570 to 1620 nm is set to the wavelength band of 1520 to 1570 nm.
An optical transmission line of the fourth arrangement of this invention is characterized in that, in addition to having the above-described first, second, or third arrangement, the dispersion value of the optical transmission line as a whole is made greater than or equal to xe2x88x921 ps/nm/km and less than or equal to 1 ps/nm/km within the 1520 to 1570 nm wavelength band.
An optical transmission line of the fifth arrangement of this invention is characterized in that, in addition to having the above-described first, second, or third arrangement, a function for compensating the wavelength dependence of the transmission loss in the wavelength band of 1570 to 1620 nm is provided.
An optical transmission line of the sixth arrangement of this invention is characterized in that, in addition to having the above-described fourth arrangement, a function for compensating the wavelength dependence of the transmission loss in the wavelength band of 1570 to 1620 nm is provided.
An optical transmission line of the seventh arrangement of this invention is characterized in that a positive dispersion optical fiber, with which both the dispersion value and dispersion slope in a preset wavelength band within a wavelength band of 1520 to 1570 nm are positive, a negative dispersion optical fiber, which compensates the dispersion and dispersion slope of the abovementioned positive dispersion optical fiber in a wavelength band adjacent the wavelength band of 1520 to 1570 nm, and a dispersion characteristics adjustment fiber, which compensates the dispersion and dispersion slope in the abovementioned preset wavelength band of the optical fiber connection unit formed by connection of the abovementioned negative dispersion fiber and the abovementioned positive dispersion optical fiber, are connected to make the dispersion value of the optical transmission line as a whole greater than or equal to xe2x88x921 ps/nm/km and less than or equal to 1 ps/nm/km within the abovementioned preset wavelength band.
An optical transmission line of the eight arrangement of this invention is characterized in that the wavelength band adjacent the wavelength band of 1520 to 1570 nm is set to the wavelength band of 1570 to 1620 nm.
An optical transmission line of the ninth arrangement of this invention is characterized in that, in addition to having the arrangement of the above-described first, second, third, sixth, seventh, or eighth arrangement, the abovementioned positive dispersion optical fiber has a positive dispersion in at least the wavelength band of 1520 to 1620 nm.
An optical transmission line of the tenth arrangement of this invention is characterized in that, in addition to having the arrangement of the above-described fourth arrangement, the abovementioned positive dispersion optical fiber has a positive dispersion in at least the wavelength band of 1520 to 1620 nm.
An optical transmission line of the eleventh arrangement of this invention is characterized in that, in addition to having the arrangement of the above-described fifth arrangement, the abovementioned positive dispersion optical fiber has a positive dispersion in at least the wavelength band of 1520 to 1620 nm.
An optical transmission line of the twelfth arrangement of this invention is characterized in that, in addition to having the arrangement of the above-described first, second, third, sixth, seventh, or eighth arrangement, the abovementioned negative dispersion optical fiber has a negative dispersion in at least the wavelength band of 1520 to 1620 nm.
An optical transmission line of the thirteenth arrangement of this invention is characterized in that, in addition to having the arrangement of the above-described fourth arrangement, the abovementioned negative dispersion optical fiber has a negative dispersion in at least the wavelength band of 1520 to 1620 nm.
An optical transmission line of the fourteenth arrangement of this invention is characterized in that, in addition to having the arrangement of the above-described fifth arrangement, the abovementioned negative dispersion optical fiber has a negative dispersion in at least the wavelength band of 1520 to 1620 nm.
A negative dispersion optical fiber of the first arrangement of this invention is characterized in being used in any of the above-described optical transmission lines of the first through fourteenth arrangements, having the dispersion value at an arbitrary single wavelength in the preset wavelength band within the wavelength band of 1570 to 1620 nm being set greater than or equal to xe2x88x9275 ps/nm/km and less than or equal to xe2x88x9215 ps/nm/km, and being made negative in the value of the dispersion slope in the abovementioned preset wavelength band and thereby provided with the characteristics of lowering the dispersion value and dispersion slope in the abovementioned preset wavelength band of a positive dispersion optical fiber installed in the abovementioned optical transmission line.
A negative dispersion optical fiber of the second arrangement of this invention is characterized in that, in addition to having the arrangement of the above-described negative dispersion optical fiber of the first arrangement, the transmission loss at an arbitrary single wavelength in the preset wavelength band within the wavelength band of 1570 to 1620 nm is set to 0.27 db/km or less, the polarization dependence loss is set to 0.15 ps/km1/2 or less, and the mode field diameter is set to 5.5 xcexcm or more to provide bending loss characteristics that enable the fiber to be made into a cable.
A negative dispersion optical fiber of the third arrangement of this invention is characterized in that, in addition to having the arrangement of the above-described negative dispersion optical fiber of the first or second arrangement, a center core with an outer diameter of a, a side core, which surrounds the center core and has an outer diameter of b, and a cladding, which surrounds the side core, are provided, and when the specific differential refractive indices of the abovementioned center core and side core based on the refractive index of the abovementioned cladding are given as xcex941 and xcex942, respectively, the value of a/b is set within the range, 0.4 to 0.55, the value of xcex942/xcex941 is set within the range, xe2x88x920.45 to xe2x88x920.30, xcex941 is set within the range, 1 to 1.4%, and the value of a is set within the range 10.5 to 14.0 xcexcm.
A negative dispersion optical fiber of the fourth arrangement of this invention is characterized in that, in addition to having the arrangement of the above-described negative dispersion optical fiber of the first or second arrangement, a center core with an outer diameter of a, a first side core, which surrounds the center core and has an outer diameter of b, a second side core, which surrounds the first side core and has an outer diameter of c, and a cladding, which surrounds the second side core, are provided, and when the specific differential refractive indices of the abovementioned center core, first side core, and second side core based on the refractive index of the abovementioned cladding are given as xcex941, xcex942 and xcex943, respectively, the value of xcex941 is set within the range, 0.9 to 1.5%, the value of xcex942 is set within the range, xe2x88x920.5 to xe2x88x920.2%, the value of xcex943 is set within the range, 0.2 to 0.3%, a, b, and c are set to satisfy a less than b less than c, the value of a:b:c is set within the range, 1:2 to 2.5:2.5 to 3.5, and the value of c is set within the range 13 to 19 xcexcm.
An optical transmission system of the first arrangement of this invention is characterized in that an optical transmission line of any of the above-described first through fourteenth arrangements is installed and when an optical signal is to be transmitted along this optical transmission line, the optical signal is transmitted upon dividing the optical signal into an optical signal of a first preset wavelength band within a wavelength band of 1570 to 1620 nm and an optical signal of a second preset wavelength band within a wavelength band adjacent the wavelength band of 1570 to 1620 nm.
An optical transmission system of the second arrangement of this invention is characterized in that, in addition to having the arrangement of the above-described optical transmission system of the first arrangement, the optical signal of the first preset wavelength band and the optical signal of the second preset wavelength band are transmitted in mutually opposite directions.
An optical transmission system of the third arrangement of this invention is characterized in that an optical transmission line of any of the above-described second through fourteenth arrangements is formed by connecting the positive dispersion optical fiber, the dispersion characteristics adjustment optical fiber, and the negative dispersion fiber in that order, the terminal part of the positive dispersion optical side of this optical transmission line is used as the input terminal part for an optical signal of a first preset wavelength band within a wavelength band of 1570 to 1620 nm, the terminal part of the negative dispersion optical side of the optical transmission line is used as the input terminal part for an optical signal of a second preset wavelength band within a wavelength band adjacent to 1570 to 1620 nm, and the optical signal of the first preset wavelength band and the optical signal of the second preset wavelength band are transmitted in mutually opposite directions.
An optical transmission system of the fourth arrangement of this invention is characterized in that, in addition to having the arrangement of the above-described optical system of the first, second, or third arrangement, the respective dispersion values and dispersion slopes of the optical fibers that comprise the optical transmission line and the transmission directions of the optical signals are determined so that neither the cumulative dispersion of the optical transmission line with respect to the optical signal of the first preset wavelength band nor the cumulative dispersion of the optical transmission line with respect to the optical signal of the second preset wavelength band will be zero from the point immediately after optical signal input to the termination of the optical transmission line.
With an optical transmission line of any of the first through third arrangements of this invention, the construction of a low-dispersion WDM optical transmission line that is suited for high-speed, large-capacity transmission is enabled by the making of a small wavelength dispersion value at a preset wavelength within the wavelength band of 1570 nm to 1620 nm.
Also, with an optical transmission line of the fourth arrangement, the construction of a low-dispersion WDM optical transmission line that is suited for high-speed, large-capacity transmission is enabled by the making of a small wavelength dispersion value at a preset wavelength within the wavelength band of 1520 nm to 1620 nm.
Furthermore, an optical transmission line of the fifth or sixth arrangement enables the wavelength dependence of the loss to be improved in addition to providing the above-described effects. Thus with the optical transmission line of the fifth or sixth arrangement of this invention, it also becomes possible at the same time to satisfy the requirement of having low dispersion characteristics in the range, 1520 nm to 1570 nm.
Furthermore, with an optical transmission line of the seventh or eighth arrangement of this invention, the construction of a low-dispersion WDM optical transmission line that is suited for high-speed, large-capacity transmission is enabled by the use of a negative dispersion optical fiber, the characteristics of which have been optimized for a wavelength band adjacent to the wavelength band of 1520 nm to 1570 nm, to make the wavelength dispersion value small at a preset wavelength within the: wavelength band of 1520 nm to 1570 nm.
Furthermore, with an optical transmission line of any of the ninth through fourteenth arrangements, the dispersion of the positive dispersion optical fiber or the negative dispersion optical fiber that comprises an optical transmission line of any of the first through eighth arrangements is specified. An optical transmission line of any of the ninth through fourteenth arrangements thus enables an optical transmission line of any of the first through eighth arrangements to be arranged accurately using a positive dispersion optical fiber or a negative dispersion optical fiber as described in the corresponding arrangement.
The negative dispersion optical fiber that is used in an optical transmission line of this invention can specifically realize the arrangement of a negative dispersion optical fiber that is suitable for the above-described transmission lines and, for example, can provide a line-type dispersion compensation optical fiber of low nonlinearity.
And by applying for example the abovementioned line-type dispersion compensation optical fiber as a negative dispersion optical fiber and thereby providing a means for WDM transmission in the L-band, maximal use can be made of such characteristics in L-RDF as low nonlinearity, low loss, and enabling of compensation in the L-band at low PMD. WDM transmission not only in the L-band but in the L-band+C-band as well is enabled by the use of the abovementioned line-type dispersion compensation optical fiber as a negative dispersion optical fiber.
Furthermore, the possibilities of WDM transmission in the L-band are expanded by the provision of an optical transmission line with which WDM transmission in the L-band can be performed using a conventional RDF.
Furthermore, since an optical transmission system that uses an optical transmission line of this invention is an optical transmission system that uses an optical transmission line that exhibits the above-described excellent effects, it can be made an excellent optical transmission system that enables wavelength division multiplexed optical transmission.
Also, with an optical transmission system of this invention, an optical signal in a first preset wavelength band within the wavelength band of 1570 to 1620 nm and an optical signal in a second preset wavelength band within a wavelength band adjacent to the wavelength band of 1570 to 1620 nm are set. Since the optical transmission system of this invention transmits an optical signal to be transmitted upon dividing the signal into the abovementioned optical signals of the first preset wavelength band and second preset wavelength band, the optical signal of the first preset wavelength band can for example be amplified by an optical amplifier for amplification in that wavelength band and the optical signal of the second preset wavelength band can be amplified by an optical amplifier for amplification in that wavelength band. An optical transmission system of this invention can thus perform wavelength division multiplexed transmission, etc. accurately.
Furthermore, with an optical transmission system of the second or third arrangement of this invention, since the optical signal of the first preset wavelength band and the optical signal of the second preset wavelength band are transmitted in mutually opposite directions, the overlapping of the signal optical power of the first preset wavelength band with the signal optical power of the second preset wavelength band can be repressed, to thereby restrict the occurrence of nonlinear phenomena, etc.
Also, with an optical transmission system of the second or third arrangement of this invention, an optical system with which neither the cumulative dispersion of the optical transmission line with respect to the optical signal of the first preset wavelength band nor the cumulative dispersion of the optical transmission line with respect to the optical signal of the second preset wavelength band will be zero from the point immediately after optical signal input to the termination of the optical transmission line can be arranged as in an optical transmission system of the fourth arrangement of this invention. By arranging an optical transmission system thus, since the optical signals of the first and second preset wavelength bands will not have to pass through a point at which the dispersion at the signal optical wavelength is zero, waveform distortion due to nonlinear phenomena can be restricted even more concretely.